Sen. Tom Coburn (R-Okla.), one of the Senate’s most outspoken advocates for deficit reduction, also aired his misgivings over the McConnell plan.

Rubio said on CBS’s “Face the Nation” that he would not vote for the proposal as he currently understands it.

“The way the deal is currently structured right now … it gives the president the ability to raise the debt limit, but as I’ve said already on the program, the debt limit’s not really the problem here. The problem is the debt,” Rubio said.

Citing a report by Standard and Poor’s, Rubio warned the nation’s credit rating would be in danger if Congress does not take significant steps to reduce the deficit. He said the McConnell plan, which is under negotiation with Senate Majority Leader Harry Reid (D-Nev.), would not safeguard the nation’s credit rating.

“I don’t believe this plan, as it’s been outlined to me, is a credible solution to our debt problem,” Rubio said.

Coburn said he is unlikely to support the McConnell-Reid plan but has not made a final decision.

“I am only going to support something that actually solves the problem, and if we don't solve the problem, and not the political problem ... I am not going to support it,” he said on “Face the Nation” Sunday morning.

Coburn said McConnell’s plan ignores the nation's fiscal crisis for the sake of political expediency. Coburn, who doesn’t plan to run for reelection, says he doesn’t care about politics anymore.

“I think the McConnell plan is more of Washington not taking responsibility, it is a great political plan, it takes the pressure off all of the politicians, but allows us to pass a debt limit without making the hard choices that this country has to make,” he said.

He said he intends the plan to be a menu of policy options for lawmakers to pick from to reduce the deficit.

Rubio and Coburn join Sens. Jim DeMint (R-S.C.) and Mike Lee (R-Utah), members of the Senate Tea Party Caucus, as critics of McConnell’s contingency plan. Rubio and Coburn are not members of that caucus, although both are popular among Tea Party voters.

DeMint last week called the plan an abdication of responsibility for Republicans.

“It basically says you can raise the debt limit, but we don’t have to vote for it,” he said on the Scott Hennen radio show. “It’s like leaving the jail door open and looking the other way, then saying it’s not our fault.”

McConnell has proposed legislation that would authorize President Obama to increase the debt limit by $2.5 trillion in three tranches. Congress could block the action only by passing a resolution of disapproval but the president could veto such a measure and it would require two-thirds of both chambers to override him.

Senate Republican Whip John Kyl (Ariz.) defended the plan on ABC’s “This Week” as potentially necessary to avoid a national default.

He said Republicans would not agree to a deficit-reduction deal that raises tax revenue by ending a variety of tax breaks, something Obama has demanded.

“If there's no other way to reach some kind of savings agreement, then at the end of the day, Republican leaders have made it clear that we will not be the ones who put the government into default,” Kyl said.

The political rationale for the McConnell plan is that it would put the responsibility of raising the debt ceiling on the White House and spare Republicans the onus of voting to increase borrowing authority.

Lee, however, said last week that voters would still likely blame Republicans for giving Obama nearly unilateral authority to raise the debt limit.

“The public would justifiably, correctly understand us as voting to support [raising the debt limit] and would not be thrown off the scent of our support for that simply by saying we weren’t voting to do it,” he told reporters.